The design of railway hopper cars is governed by three main requirements. First, the fully loaded weight of a 125 ton car must not exceed 315,000 lbs. Thus to maximize useful, load car designers try to minimize car weight. At present an empty grain hopper steel car may typically weigh about 63,000 lbs., such that lading in excess of 50,000 lbs. is permissible. Second, the car must withstand a draft load of 630,000 lbs. Third, the car must not buckle under buff loads of 650,000 to 1,000,000 lbs. when slowing or stopping. Under the first, dead weight, loading condition the car may be modelled as a simply supported hollow beam carrying a distributed vertical load in excess of 50,000 lbs., with a corresponding bending moment distribution. Under the second, tensile draft, and third, compressive buff, loading conditions the car is like a column, taking tensile and compressive loads.
The general structure of contemporary curved-sided hopper cars can be idealized as a load bearing monocoque in the form of a hollow, downwardly opening, generally C-shaped, thin walled, column. At each column end, the load is transferred through a transition structure from the shell into a stub sill and coupler by which the railcar is connected to the next rail car. The challenge in designing the structure for a hopper car, in general, is to reduce the mass of the thin shell, and any supporting structure, to a minimum while still maintaining the structural integrity required to withstand the given loads, and to transfer those loads between the couplers and the body shell. When the shell is made too thin it fails in compression due either to global buckling of the structure, or to the local buckling phenomenon of wrinkling. In such a hollow shell structure, the ability to resist the compressive buff load, without buckling, requires that the principle longitudinal structural components of the car, those being the roof and side walls, work together as a single integrated structure.
One way to reduce the weight of the car is to reduce the thickness of the roof. The thickness of the roof of a typical hopper car is commonly less than 3/16". Given a railcar length of roughly 60 feet and width of roughly 10 feet, the roof may be considered a thin shell structure. Under vertical loading conditions of the car, this thin shell structure is exposed to a compressive load, with a consequent tendency toward buckling or wrinkling. This tendency is increased when a compressive longitudinal load is also applied to the car.
In the past, hopper car roofs have been given an outwardly bulging curved panel form to resist buckling, and have been supported by internal bulkheads or partition sheets, such as disclosed in U.S. Pat. No. 4,275,662 of Adler, issued Jun. 30, 1981. For example, a three hopper rail car generally has two end walls and two intermediate partitions leaving three roof spans each having a length of 15 to 20 feet. The roof is supported along its outboard edges by top chord members frequently in the form of a closed hollow section as depicted, for example, in FIG. 2 of U.S. Pat. No. 4,275,662.
In U.S. Pat. No. 4,377,058 of Hallam et al., issued Mar. 22, 1983 partial, reinforced internal stiffeners, shown as web assemblies 34 and 36, extend internally across the full width of the car and maintain the curvature of the roof In general, internal fittings, and particularly internal welds, tend to be avoided if possible. First, internal welding tends to be more difficult. Second, each additional fitting creates one or more niches in which foodstuffs may collect and rot. Third, it is generally better to leave the inside of the hopper free of obstructions. Where stiffeners are used a common goal is to obtain adequate strength without adding unnecessary weight.
The unsupported spans of hopper car roofs between end walls and bulkheads have a tendency to deflect. In particular, rapid unloading of grain hoper cars is known to cause a partial vacuum inside the car which tends to draw the roof inward. This is more pronounced in grain hopper cars having a continuous, central, longitudinally extending, trough opening. It tends to cause the arcuate shape of the roof section to flatten. This problem worsens as the thickness of the roof material decreases. The central trough may be bordered by a coaming, and the deflection of the roof may tend not only to cause the coaming to deflect, but may also tend to twist the coaming and reduce its ability to strengthen the structure. Consequently as roof thickness is reduced to lower the weight of the car it is desirable to reinforce the roof so that it provides resistance to buckling and to deflection under internal vacuum comparable to a thicker un-reinforced roof It is also advantageous to provide stiffening to maintain a natural frequency comparable to previous roofs, as vibration remains a significant factor in railcar design generally.
In general, it would be advantageous to have, and there has been a long felt need for, an improved hopper car shell structure. To that end, it would be advantageous to have improved reinforcement of a hopper car roof.